Similarity between the room transfer functions (RTFs) of symmetric loudspeaker pairs in a multichannel audio system1, e.g., left and right in a stereo system, or front left and front right, and surround left and surround right in a 5.1 surround system, is a basic requirement for correct sound reproduction [8, 11]. All equalization methods, which aim at attaining the same target response for two loudspeaker channels individually, would, ideally, obtain similarity between the channels as a byproduct. However, unless the listening environment is a perfectly symmetric room with respect to the considered loudspeaker pair, and the loudspeakers are identical, this is not a realistic outcome. Hence, if similarity between RTF-pairs is crucial, as is the case in, e.g., a stereo setup, then it is desirable that a RTF equalization design also takes similarity into account. 1In other words, symmetric loudspeaker pairs with respect to sound reproduction standards. The actual loudspeaker placement may differ from standard recommendations and is not required to be symmetric.
In general, multichannel sound reproduction always has the problem of identity: As discussed in [20, 8], for the example of stereophonic sound reproduction, exact reproduction of recorded sound in other than the genuine recording environment, by means of two loudspeakers, must be considered an impossible task. Multichannel systems with more than two channels may overcome this problem to some extent, nevertheless they still suffer from this limitation. On the other hand we know that, irrespective of the recording techniques used to create the source material, the end product of recording, mixing and mastering multichannel audio material always is a number of audio signals, e.g. two channels for stereo or six channels for 5.1 surround. The resulting perceived sound image is defined by the amplitude and phase content of those signals and their relation to each other [9, 21, 7, 11]. The mixing of the sound image is an artistic part of the production process, and by means of, e.g., microphone techniques, signal mixing and additional sound effects, the resulting sound image is created by the recording engineer [2]. Having said this, we believe that an optimal equalizer design should not strive to attain the original sound image, since this may have been altered significantly by the recording engineer anyway. Instead, as the listening experience of any equalized sound system equals, at best, the listening experience of the recording professional in the recording studio, it should strive to attain the sound image as intended by the recording engineer.
Therefore, the listening conditions in the recording studio and the psychoacoustic principles of multichannel sound reproduction are of importance. Recording studios usually constitute controlled listening environments, consisting of control rooms with symmetric loudspeaker setups with respect to the room and listening position [10, 3, 11]. In these controlled environments we can assume the RTFs of loudspeaker pairs to be fairly similar. This in accordance with [8, 21], where it is pointed out that reproducing the intended stereo image of stereo recordings requires equal intensity and equal acoustic transfer functions from the input to the two loudspeakers to the listening region. Unlike the recording professionals, the typical audience does not listen in such controlled environments. Thus, stereo or surround reproduction in, e.g., consumer homes can be assumed to severely suffer from flawed sound image reproduction.